Crystallization and Stabilization in the Synthesis of Microcrystalline Alpha Alane

ABSTRACT

Systems and methods for producing microcrystalline alpha alane are provided herein. An exemplary process comprises the elimination of the crystallization aid lithium borohydride through the use of excess lithium aluminum hydride or sodium borohydride. Further exemplary processes comprise methods for passivating microcrystalline alpha alane using a weak acid in a nonaqueous solvent solution.

CROSS REFERENCE TO RELATED APPLICATIONS

This non-provisional U.S. patent application is a divisional of U.S.patent application Ser. No. 15/184,962, filed on Jun. 16, 2016, which isa continuation-in-part of U.S. patent application Ser. No. 13/671,090,filed on Nov. 7, 2012, which claims priority benefit of provisional U.S.patent application Ser. No. 61/556,738, filed on Nov. 7, 2011. Thispatent application also claims priority benefit of provisional U.S.patent application Ser. No. 62/181,120, filed on Jun. 17, 2015, andprovisional U.S. patent application Ser. No. 62/181,129, filed on Jun.17, 2015. All of the aforementioned disclosures are hereby incorporatedby reference herein in their entireties including all references citedtherein.

This application is related to U.S. patent application Ser. No.09/823,379 filed on Mar. 29, 2001, now issued as U.S. Pat. No. 6,617,064issued Sep. 9, 2003, and entitled “Stabilized Aluminum HydridePolymorphs”. This application is also related to U.S. patent applicationSer. No. 09/334,359 filed on Jun. 16, 1999, now issued as U.S. Pat. No.6,228,338 issued May 8, 2001 and entitled “Preparation of AluminumHydride Polymorphs, Particularly Stabilized α-ALH₃”. The disclosures ofthese patents are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present technology may be generally described comprising methods forproducing microcrystalline alpha alane.

BACKGROUND

Economical production of aluminum hydride (AlH₃) or “alane” depends onan approach that combines aluminum with hydrogen in a manner that isenergy efficient and practical. However, the rate of direct reactionbetween pure aluminum and hydrogen is very slow. A major barrier to thisreaction is that little change in enthalpic energy (ΔH_(f)=−2.37kcal/mol AlH₃) occurs in the transformation of elemental aluminum andhydrogen to aluminum hydride. The ordered nature of the crystallinealuminum metal also inhibits reaction with hydrogen. Another barrier isthat the aluminum oxide (Al₂O₃) coating that forms on the surface ofaluminum when it comes in contact with air, reduces or limits thesurface area of the reactive aluminum and inhibits the reaction withhydrogen.

Methods for microcrystalline alane synthesis are inefficient forproducing large quantities of alane. These methods are problematic whenproduction of a specific alane polymorph is required, such as alphaalane (α-alane). The large amounts of solvent required as described inthe patent literature for the synthesis of the alpha polymorph of alanehinder the large-scale production of this material by these routes.Material and capital equipment costs can be reduced by a dramaticreduction in solvent for this process.

SUMMARY

Various embodiments of the present disclosure may be directed to amethod for producing alpha alane. An exemplary method may compriseadding lithium aluminum hydride to a solvent solution of aluminumtrichloride and an ether to produce alane etherate (AlH₃.Et₂O) andlithium chloride. The alane etherate may be heated in the presence of anaromatic solvent to obtain microcrystalline alpha alane.

According to additional exemplary embodiments, the present disclosuremay be directed to a method for producing alpha alane. An exemplarymethod may comprise adding lithium aluminum hydride, aluminumtrichloride, and an ether to a solvent to form a solvent solution. Anexcess of lithium aluminum hydride may be added to the solvent solution,the excess lithium aluminum hydride acting as a crystallization aid inthe conversion of alane etherate to alpha alane, a preferable alanestructural polymorph. The lithium chloride precipitate may be filteredfrom the alane etherate solution, and at least a portion of the solventmay be removed from the alane etherate solution, forming an alaneetherate concentrate. A mixture may be created comprising the alaneetherate concentrate, an ether and an aromatic solvent. The mixture maybe heated to convert the alane etherate to obtain microcrystalline alphaalane.

According to still further exemplary embodiments, the present disclosuremay be directed to a method for producing alpha alane. An exemplarymethod may comprise adding lithium aluminum hydride, aluminumtrichloride, and an ether to a solvent to form a solvent solution.Sodium borohydride may be added to the solvent solution to form lithiumborohydride, the lithium borohydride acting as a crystallization aid inthe conversion of alane etherate to alpha alane, a preferable alanestructural polymorph. The lithium chloride precipitate may be filteredfrom the alane etherate solution, and at least a portion of the solventmay be removed from the alane etherate solution, forming an alaneetherate concentrate. A mixture may be created comprising the alaneetherate concentrate, an ether, and an aromatic solvent. The mixture maybe heated to convert the alane etherate to obtain microcrystalline alphaalane.

According to yet further exemplary embodiments, the present disclosuremay be directed to a method for stabilizing aluminum hydride. Anexemplary method may comprise producing aluminum hydride. The aluminumhydride may be placed in a nonaqueous solvent. A weak acid solution maybe added to the nonaqueous solvent and the aluminum hydride may beallowed to contact the weak acid solution for a period of time to formpassivated aluminum hydride. The passivated aluminum hydride may beseparated from the solvent and weak acid solution. The passivatedaluminum hydride may be dried.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present technology are illustrated by theaccompanying figures. It will be understood that the figures are notnecessarily to scale and that details not necessary for an understandingof the technology or that render other details difficult to perceive maybe omitted. It will be understood that the technology is not necessarilylimited to the particular embodiments illustrated herein.

FIG. 1 is a flow diagram of an exemplary process for producingmicrocrystalline α-alane.

FIG. 2 is a flow diagram of an exemplary process for producingmicrocrystalline α-alane.

FIG. 3 is a flow diagram of an exemplary process for producingmicrocrystalline α-alane.

FIG. 4 is a flow diagram of an exemplary process for producingmicrocrystalline α-alane.

FIG. 5 is a flow diagram of an exemplary method for producingmicrocrystalline α-alane.

FIG. 6 is a flow diagram of an exemplary method for producingmicrocrystalline α-alane.

FIG. 7 is a flow diagram of an exemplary method for producingmicrocrystalline α-alane.

FIG. 8 is a flow diagram of an exemplary method for stabilizing aluminumhydride

FIG. 9 illustrates an x-ray diffraction analysis of a microcrystallineα-alane product produced by the process of FIG. 1.

FIG. 10 is a flow diagram of an exemplary process for producingmicrocrystalline α-alane.

DETAILED DESCRIPTION

While this technology is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail several specific embodiments with the understanding that thepresent disclosure is to be considered as an exemplification of theprinciples of the technology and is not intended to limit the technologyto the embodiments illustrated.

It will be understood that like or analogous elements and/or components,referred to herein, may be identified throughout the drawings with likereference characters. It will be further understood that several of thefigures are merely schematic representations of the present technology.As such, some of the components may have been distorted from theiractual scale for pictorial clarity.

Additionally, ranges described or claimed herein are inclusive of theirend points. Moreover, the end points are inclusive of suitablefluctuations allowing for reasonable and approximate values that fallnear end points.

As used herein, the term “alane” refers to aluminum hydride, having theformula AlH₃, and includes all the polymorphs such as α-AlH₃, α′-AlH₃,δ-AlH₃, and the like.

The alpha polymorph of aluminum hydride (AlH₃, or alane), also referredto as α-alane, is a stable compound that may be used as a hydrogensource in fuel cells and batteries, among other applications. It may beproduced in a 2-stage chemical reaction. In the first stage reaction,alane etherate is created: AlCl₃+3LiAlH₄+Et₂O→4AlH₃·Et₂O+3LiCl. In thesecond stage reaction, α-alane is generated by a thermal conversion todrive off the ether adduct to yield a solid form of alane:AlH₃.Et2O→α-AlH3. While various forms of alane exist, it may be criticalto yield the alpha form, as other forms of alane are less stable. Thetwo stages of the reaction must be carried out carefully, or thereactions will yield either just Al metal, or a mixture of AlH₃ with Almetal, which is difficult to handle and unstable. Furthermore,substantially pure α-alane is needed to be able to do a later step ofpassivation so that the alane can be used as a stable fuel source,especially for a fuel cell.

There are a several different methods for producing α-alane. One is asolution process which involves the use of a large volume of solvents.This method produces alane in batches, and is thus an expensive andtime-consuming process. Various solvents may be used for carrying outthe reaction, such as diethyl ether and toluene. Additionally, hotsolvents may be used by various heating methods. Nucleation occurs inthis process to facilitate the formation of the alane. Careful controlof the stirring method of the material in the reactor, ether content inthe reactor, heating profile and heating method, along with use ofadditives are needed to produce substantially pure α-alane. Otherparameters also need to be carefully controlled to effect a properreaction of materials to generate substantially pure α-alane in a stableform.

Another method for producing α-alane is in a slurry process. To remedyuneven heating and the aforementioned resultant deleterious effects, thesolids may be combined with a solvent such as toluene to produce aslurry. When heat is applied to the slurry the solvent allows the heatto be evenly distributed throughout the slurry, reducing thedecomposition of the alane etherate precursor into aluminum andalternate alane polymorphs.

In the slurry process, the materials in the reactor are much more highlyconcentrated, and thus significantly less solvent is needed to carry outthe reactions to completion. This allows for production of largequantities of microcrystalline alane at reduced material and capitalcosts. Alane is still produced in batches. The heating methods andheating profile of this process must also be carefully controlled, alongwith stirring method, ether content, and other parameters. Nucleationoccurs in this process to facilitate the formation of the alane.Precipitation aids and methods may also be employed to facilitate theproduction of the α-alane, along with crystallization aids. Passivationmethods may further be employed to produce a stable form of the α-alane.The microcrystalline form of α-alane comprises an enhanced surface area,which provides for enhanced reactivity of the alane. This enhancedreactivity may be reduced via passivation.

Passivation may occur by introducing the microcrystals into anon-aqueous solvent such as toluene. In some instances, the slurry ofmicrocrystals and non-aqueous solvent may be combined into a lowconcentration acidic solution such as between 1 to 5% hydrochloric acid.The microcrystals may also be added directly to the acid solution. Thisprocess passivates the surface of the microcrystals by creating analuminum oxide coating on the surface of the alane. Also, thehydrochloric acid destroys more reactive alane polymorphs as well asresidual lithium borohydride and lithium aluminum hydride. Passivatedalane is safer to handle than the more reactive alane. Additionally,passivated alane has a longer shelf life compared to un-passivated alanethat also can contain impurities that can catalyze the decomposition ofthe alane and are highly reactive to water and ambient air.

A third method for producing α-alane is in a continuous process. In thisprocess, one or both stages of the reactions for producing α-alane mayoccur in a continuous flow reactor as opposed to a batch reactor. Thisallows for faster production of the alane along with fewer raw materialsto be used (i.e., not as much solvent needed).

A continuous process for producing α-alane may allow for two-stagepassivation. Similar to the solution process and the slurry process,careful control of heating methods and heat profile are needed in thereactor, along with other parameters. Various methods for removing etherfrom the process may be employed.

A fourth method of producing α-alane is in an electrochemical cell. Inthis method, an electric field is applied across a solution, causing thepositive ions to move toward the negative side of the field and thenegative ions to move toward the positive side. The two half-cells of anelectrochemical cell are connected by a cell separator that allows ionsto move between the half-cells but prevents mixing of the electrolytes.The separator may be a membrane, to selectively allow certain compoundsto pass between sides. In a typical electrochemical cell used to producealane etherate, the rate of alane production is limited by the currentflow through the cell.

Generally speaking, various embodiments may comprise systems andprocesses for synthesizing microcrystalline alane, preferably the alphapolymorph (α-alane). In some embodiments, exemplary processes maycomprise combining excess lithium aluminum hydride (LiAlH₄) with anamount of aluminum trichloride (AlCl₃) in an ether solvent to producelithium aluminum hydride, lithium chloride, and alane etherate. Next,the lithium chloride may be separated by filtration leaving acomposition of ether, alane etherate and lithium aluminum hydride. Thismixture may be further processed to remove ether, and the resultingsolid may be heated to separate the alane from the alane etherate.

It is noteworthy that alane etherate is sensitive to temperature. Thus,uneven or incomplete heating may result in the decomposition of alaneetherate into aluminum and hydrogen gas, which produces a potentiallyflammable product. Additionally, heating may cause the alane etherate totransform into various polymorphs. The transformation of alane into itsvarious polymorphs is undesirable when α-alane synthesis is the desiredgoal.

To remedy uneven heating and the aforementioned resultant deleteriouseffects, the solids may be combined with a solvent such as toluene toproduce a slurry. When heat is applied to the slurry the toluene allowsthe heat to be evenly distributed throughout the slurry, reducing thedecomposition of alane etherate into aluminum and alternate polymorphs.

Production of α-alane

FIG. 1 is a flow diagram of an exemplary process 100 used for alanesynthesis according to various embodiments. At steps 105 and 110, thesynthesis involves the addition of a solution of aluminum trichloride(AlCl₃) to lithium aluminum hydride (LiAlH₄) in an ether, such asdiethyl ether, to generate at step 115 alane etherate (AlH₃.Et₂O) insolution and lithium chloride precipitate. The solubility of alaneetherate has an inverse relationship to temperature, and keeping thereaction temperature to within a range of approximately −5° C. to 0° C.reduced the observed premature crystallization at ambient temperature.The concentration of the AlH₃.Et₂O was approximately 0.8 M duringfiltration at step 120.

Following filtration at step 120, a solution of lithium borohydride(LiBH₄) dissolved in 1.0 M of diethyl ether (step 125) is added to thefiltered mixture. The LiBH₄ may act as an optionally addedcrystallization aid by improving the size and shape of the alanecrystals. The next step involves the removal of most of the diethylether solvent by vacuum distillation at step 130. After removal ofapproximately all visible solvent and pumping under high vacuum for anadditional period of time (which in some instances may range fromapproximately two to three hours), the remaining solid material ofapproximate composition LiAlH₄/4AlH₃.1.2Et₂O/LiBH₄ may be a bright whitepowder material. This solid material may be ground at step 135 andtransferred into a separate flask for heating. Several separateexperiments were preformed where only 70-95% of the diethyl ether wasremoved (indicated as “Partial Removal” in FIG. 1) giving a slurry ofthe LiAlH₄/4AlH₃.Et₂O/LiBH₄ mixture in a minimal amount of diethylether.

At step 145, heating of this solid mixture to a temperature that fallswithin a range of approximately 60° C. to 65° C. for a period of time ofapproximately four hours and in the presence of a vacuum, converts thealane etherate into α-alane and other products.

Empirical data suggests that heating of the solid mixture resulted inuneven heating, which leads to mixtures of products being formed. Asmentioned above, uneven heating may result in decomposition of the alaneetherate into aluminum and hydrogen gas, as well as the formation ofvarious alternate alane polymorphs.

Thus, the process 100 may include a step of combining the solid (fromstep 135) or diethyl ether slurry mixture (from step 130) with anaromatic solvent such as toluene (C₇H₈) at step 140 prior to heating atstep 145. Other aromatic solvents such as benzene, ethylbenzene, xyleneand mixed xylenes, naphthas, and mixtures thereof may also be used. Thearomatic solvent may act as a heat sink and more optimal distribution ofsolids throughout the solvent may allow for better distribution of heatthroughout the sample. Advantageously, even distribution of heat duringthe heating cycle may provide efficient transformation of the alaneetherate into α-alane thereby avoiding the generation of hot spots,which can degrade the alane etherate precursor.

According to some embodiments, the slurry may be heated to a temperaturethat falls within a range of approximately 72° C. to 80° C., over aperiod of time of approximately three to six hours. During the heatingprocess alane etherate may be transformed into α-alane. Heating at lowertemperatures may result in the decomposition of alane etherate and thegeneration of alternate alane polymorphs. It will be understood that asthe temperature increases, the length of the heating cycle may decrease.The temperature employed in this process is approximately 20° C. lowerthan used for known methods of producing microcrystalline α-alane, thussignificantly reducing energy costs of alane production.

After heating, the solid may be rinsed with an ether, such as diethylether, at step 150 which may dissolve the more soluble excess lithiumaluminum hydride and lithium borohydride. These crystallization aidesmay be reused in subsequent batches of alane production. Rinsing andfiltering of remaining lithium borohydride and lithium aluminum hydridefrom the toluene slurry, and subsequent drying at step 155, may provideα-alane in microcrystal form.

The microcrystal form of α-alane, comprises an enhanced surface area,which provides for enhanced reactivity of the alane. This enhancedreactivity may be reduced via passivation.

Passivation may occur by introducing the microcrystals into anon-aqueous solvent such as dimethoxyethane or toluene prior to theaddition of the aqueous acid. This may result in distributing the heatgenerated during the passivation process. In some instances, the slurryof microcrystals and non-aqueous solvent may be combined into a lowconcentration mineral acid solution such as between 1 to 5% hydrochloricacid at step 160. Other mineral acids or buffered solutions of theseacids may also be used in the passivation step, such as phosphoric acid(H₃PO₄), sulfuric acid (H₂SO₄), boric acid (H₃BO₃), hydrofluoric acid(HF), hydrobromic acid (HBr), hydroiodic acid (HI), and mixturesthereof.

The microcrystals may also be added directly to the acid solution. Afterwashing and filtering at step 160, the microcrystals may be dried atstep 165 producing the passivated α-alane final product. This processmay passivate the surface of the microcrystals by creating an aluminumoxide coating on the surface of the alane. Also, the acid may passivateor destroy more reactive alane polymorphs as well as any aluminum metal,residual lithium borohydride and lithium aluminum hydride. Passivatedalane is safer to handle than the more reactive alane. Additionally,passivated alane has a longer shelf life compared to un-passivated alanethat also can contain impurities that are highly reactive to water andambient air.

FIG. 10 is a flow diagram of an exemplary process used for alanesynthesis according to various embodiments. The process may be acontinuous flow production process, composed of three reactors. In thefirst reactor (also referred to as a LiAlH₄ reactor 1010), AlH₄, LiCland THF are fed into the reactor via stream 1005. It should be notedthat even though the figure only depicts these components and streams,other materials may be also be present in the reactor or the process.From the LiAlH₄ reactor, solids are separated in separator 1015, andNaCl and LiAlH₄ are generated in streams 1020 and 1025, respectively.

The generated LiAlH₄ stream 1025 may be fed into an etherate reactor1030 with AlCl₃ and ether via stream 1035 to generate LiCl. Again, whileFIG. 10 only depicts certain components for simplicity, other materialsmay also be present at any step of the process. A primary reaction inthe etherate reactor is: AlCl₃+LiAlH₄+Et₂O->AlH₃.Et₂O. The resultingmaterials can be concentrated and fed into a crystallization reactor viaprocesses 1045 and 1050. A primary reaction in the crystallizationreactor 1050 is: AlH₃.Et₂O->α-AlH₃. The α-AlH₃ product may besubsequently washed, passivated (from stream 1055), separated and driedas depicted in FIG. 10.

Elimination of Lithium Borohydride Crystallization Aid

A significant portion of the cost associated with producing α-alane isthe addition of a crystallization aid, which is a material in thesolution to facilitate the crystallization from alane etherate toα-alane in the second stage reaction. When the crystallization processoccurs in a slurry instead of in a solution, the reactor can be muchsmaller. Typically, lithium borohydride is used as a crystallizationaid, which is added to the reactor after alane etherate is formed (seestep 125 of FIG. 1). The lithium borohydride improves the crystallinityof the resulting alpha alane product.

In various embodiments of the solution process as illustrated in anexemplary process 200 in FIG. 2, excess lithium aluminum hydride can beadded to the reactor at step 115, which is then carried along from thefirst reaction into the second reaction and essentially acts as thecrystallization aid in the second reaction. In this way, one can avoidthe necessity of adding lithium borohydride for the crystallizationprocess of producing aluminum hydride (note that step 125 of FIG. 1 hasbeen eliminated from FIG. 2). This is useful because elimination oflithium borohydride from the process allows more efficient recycling ofthe lithium aluminum hydride. An additional benefit is the removal ofthe expensive lithium borohydride from the crystallization process.

In further embodiments, lithium aluminum hydride can be generated fromsodium aluminum hydride in a process similar to that depicted in theLiAlH₄ reactor of FIG. 10. This is advantageous since sodium aluminumhydride is a less costly material than lithium aluminum hydride.

Additionally, in various embodiments utilizing a slurry process may beperformed without a crystallization aid. Essentially a solution processtype chemistry can be utilized, but in a slurry process. Excess lithiumaluminum hydride may be utilized alone as a method of converting alaneetherate into alane. The excess lithium aluminum hydride may be addeddirectly, or generated from sodium aluminum hydride first, in a processsimilar to the LiAlH₄ reactor depicted in FIG. 10.

In further embodiments as illustrated in an exemplary process 300 inFIG. 3, instead of adding lithium borohydride after alane etherate isformed in the reactor, sodium borohydride (NaBH₄) may be added to thereactor for the first reaction at step 115 to generate the lithiumborohydride in the reactor in situ. In the presence of lithium chloride,the sodium borohydride undergoes a cation exchange reaction to thelithium borohydride. Thus, lithium borohydride may be present in thereactor to act as a crystallization aid without having to add itdirectly. This may reduce the raw material cost to that of the sodiumborohydride instead of lithium borohydride, which is significantly moreeconomical. This allows for a more economical larger scale production ofα-alane.

Aluminum Hydride Stabilization

The terms “stabilized” or “stabilization” when used to refer to theα-alane product indicates that the product is substantially more stablethan α-alane products of the prior art (i.e., α-alane prepared withoutthe use of an acid wash workup and/or without stabilizing agents asdisclosed herein). That is, “unstabilized” α-alane may rapidly decomposeto hydrogen and aluminum, while the stabilized α-alane of the inventiondoes not. “Stability” refers to both thermal stability and stability atambient temperature. With respect to thermal stability, the “stabilized”α-alane of the invention may be less than 1% decomposed after twelvedays at 60° C., while decomposition of the unstabilized product at thatpoint may be virtually complete.

Regardless of which method is employed for the synthesis of aluminumhydride, it may be advantageous to stabilize the aluminum hydridebecause it is particularly susceptible to decomposition when exposed toair and/or water. The microcrystal form of α-alane, such as thatproduced in the slurry process, comprises an enhanced surface area,which may be 3-5 times greater than non-microcrystalline α-alane. Theincreased surface area may result in enhanced reactivity of the alane.This enhanced reactivity may be reduced via passivation. Passivatedalane is safer to handle than the more reactive unpassivated alane.Additionally, passivated alane has a longer shelf life compared toun-passivated alane that also can contain impurities that are highlyreactive to water and ambient air.

The passivation step in FIG. 1 (step 160) may utilize a hydrochloricacid solution. However, hydrochloric acid may be too aggressive whenmicrocrystals of aluminum hydride are produced, leading to breakdown ofthe alpha polymorph. Thus, as illustrated in FIG. 4, a method 400 maycomprise stabilizing the aluminum hydride by placing the aluminumhydride in a nonaqueous solvent and then adding a weak acid solution ofgenerally 1-5% (see steps 405-410 followed by steps 160-165 of FIG. 4).In some embodiments, the aluminum hydride may be combined with anonaqueous solvent such as diethyl ether or toluene at step 405 toprevent decomposition. Any nonaqueous solvent, such as aromatic orhydrocarbon solvents and mixtures thereof may be suitable for this step.

The passivation process, which converts a portion of the aluminumhydride to an aluminum oxide coating on the surface of the aluminumhydride crystals, is highly exothermic. The rate of this reactionincreases with increasing temperature, so control of the heat generatedduring the passivation reaction may be necessary to prevent an excessiveamount of aluminum hydride (the desired product) to be consumed. In step405, the nonaqueous solvent may act as a heat sink to moderate thereaction temperature and to maintain a desired reaction rate forpassivation.

At step 410, the weak acid solution may be added to the solvent-aluminumhydride suspension, forming a bi-phasic or mono-phasic solution. Invarious embodiments, the weak acid solution may comprise an acid that isweaker than hydrochloric acid. One measure of acid strength is theacid's dissociation constant (K_(a)). Thus, an acceptable weak acid maybe an acid with a K_(a) value less than that of hydrochloric acid.Another measure of acid strength is whether it partially or fullydissociates in water. According to this measure, an acceptable weak acidmay be an acid that only partially dissociated in water. Non-limitingexamples of weak acids comprise chloroacetic acid, lactic acid, maleicacid, malonic acid, nitrous acid, oxalic acid, periodic acid, phosphoricacid, phosphorous acid, o-phthalic acid, salicylic acid, sulfurous acid,tartaric acid, and mixtures thereof.

At step 160, the bi-phasic solution may be filtered to remove the solidaluminum hydride from the solution, then washed with water to removeresidual solvent and acid. The aluminum hydride may then be washed withan alcohol to remove residual water and allow more efficient drying. Thealuminum hydride may then be dried at step 165. The drying process maycomprise heating the aluminum hydride at atmospheric pressure, or undervacuum.

FIG. 5 is a flowchart of an exemplary method 500 for producing alphaalane according to various embodiments. At step 505, lithium aluminumhydride may be added to a solvent solution of aluminum trichloride andan ether to produce alane etherate and lithium chloride. The alaneetherate may be heated at step 510 in the presence of an aromaticsolvent to obtain microcrystalline alpha alane.

FIG. 6 is a flowchart of an exemplary method 600 for producing alphaalane according to various embodiments. Lithium aluminum hydride,aluminum trichloride, and an ether may be added to a solvent to form asolvent solution at step 605. At step 610, an excess of lithium aluminumhydride may be added to the solvent solution. The excess lithiumaluminum hydride facilitates the transformation of the alane etherate toalpha alane in the heating step. The lithium chloride precipitate may befiltered at step 615 from the alane etherate solution, and at least aportion of the solvent may be removed from the alane etherate solutionat step 620, forming an alane etherate concentrate. At step 625 amixture may be created comprising the alane etherate concentrate, anether and an aromatic solvent. The mixture may be heated to convert thealane etherate to obtain microcrystalline alpha alane at step 630.

FIG. 7 is a flowchart of an exemplary method 700 for producing alphaalane according to various embodiments. At step 705, lithium aluminumhydride, aluminum trichloride, and an ether may be added to a solvent toform a solvent solution. Sodium borohydride may be added to the solventsolution at step 710 to form lithium borohydride. The lithiumborohydride improves the crystallinity of the resulting alpha alaneproduct. At step 715, the lithium chloride precipitate may be filteredfrom the alane etherate solution, and at least a portion of the solventmay be removed from the alane etherate solution at step 720, forming analane etherate concentrate. A mixture may be created at step 725comprising the alane etherate concentrate, an ether and an aromaticsolvent. The mixture may be heated to convert the alane etherate toobtain microcrystalline alpha alane at step 730.

FIG. 8 is a flowchart of an exemplary method 800 for stabilizingaluminum hydride according to various embodiments. At step 805, aluminumhydride may be produced. The aluminum hydride may be placed in anonaqueous solvent at step 810. At step 815, a weak acid solution may beadded to the nonaqueous solvent and the aluminum hydride may be allowedto contact the weak acid solution for a period of time to formpassivated aluminum hydride. The passivated aluminum hydride may beseparated from the solvent and weak acid solution at step 820. Thepassivated aluminum hydride may be washed and then dried at step 825.

EXAMPLES

Processes described herein may use dramatically lower amounts of solventcompared to the current route used for synthesis of microcrystallineα-alane. Alane synthesis experiments described in greater detail hereinwere designed to provide more information on the route tomicrocrystalline α-alane. Several parameters such as temperature, time,solvent and concentration were varied to investigate their effect on theprocess.

The highest purity starting materials were utilized in these experimentsand included: (1) lithium aluminum hydride in 1.0 M diethyl ether; (2)aluminum chloride, 99.99%; and (3) lithium borohydride, 99.5%, combinedtogether in a solvent such as diethyl ether. The diethyl ether is driedfrom sodium metal. Toluene may be dried over molecular sieves.

Example 1

Alane Synthesis (20-g Batch)

This batch used a single pot 20-g scale-up reactor apparatus. Theapparatus enabled rapid filtration of alane etherate solutions, moreefficient temperature control, and the ability to work at a highersolvent load (approximately 1.2 L) required for the 20-g scale. Thesafety of the synthesis procedure was also greatly improved. Forexample, methods that use unevenly distributed heating methods mayproduce decomposed alane, which results in reactive aluminum andhydrogen gas, which are volatile materials.

The lithium aluminum hydride used for this procedure included purified95% material. The solid from the dry box was loaded into a 1 L flask anddissolved using approximately 800 mL of diethyl ether. This mixture wasthen transferred into a 2 L jacketed reactor assembly. The solution wasstirred by an overhead stirrer and cooled to approximately −8° C. usinga recirculating bath unit. The aluminum chloride (approximately 26.6 g)was dissolved in approximately 200 mL of diethyl ether. This mixture wasthen added into the reactor over a time frame of approximately fiveminutes and combined with the cooled lithium aluminum hydride solution.Lithium chloride precipitate appeared immediately and settled rapidlywhen stirring terminates.

Immediately after addition the solution was filtered into a flask underthe reactor over approximately five minutes time. The flask containingthe clear filtrate was then disconnected from the reactor unit andapproximately 4.34 g of lithium borohydride, which had beenpre-dissolved in approximately 350 mL of diethyl ether was added intothe reactor. The diethyl ether was removed via trap-to-trap distillationunder vacuum. Any remaining solvent was removed under dynamic vacuum ofapproximately 0.02 torr for a period of time of approximately seventeenhours.

The final mass of the resultant α-alane after transfer and fine grindingwas approximately 52.8 g. A 13 g portion of this material was heated at71° C. for approximately six hours as a stirred suspension in toluene(35 mL), which resulted in a light grey powder after filtration andrinsed three times with 150 mL of diethyl ether each time.

FIG. 9 illustrates an x-ray diffraction analysis showing all α-alanewith no evidence of other alane polymorphs or aluminum metal. Thus,heating of the lithium aluminum hydride, alane etherate, and lithiumborohydride mixtures as toluene slurries at a temperature ofapproximately 71° C. transforms the initially formed Γ-alane into thealpha phase with no evidence of resultant aluminum metal.

Example 2

Alane Synthesis (20-g Batch)

This reaction was performed following the above described procedure andapparatus, and produced approximately 20-g batches of α-alane. A 1.0 Mlithium aluminum hydride solution was used. The final mass after thereaction, solvent removal, drying phase, and fine grinding wasapproximately 53.3 g. This material was heated to approximately 71.4° C.for approximately six hours in toluene and gave a light-grey powderafter rinsing three times in 300 mL of diethyl ether and drying on aglass filter frit. The yield of non-passivated material wasapproximately 22.5 g. X-ray diffraction analysis showed all α-alane withno evidence of other polymorphs or aluminum metal in the mixture.Differential scanning calorimetry (DSC) and thermogravimetric analysis(TGA) both show an aliquot of the non-passivated material, which shows asingle endothermic DSC peak at approximately 169° C. and a weight lossof approximately 8.8% starting at 151° C.

Example 3

Alane Synthesis (Heating in Minimal Diethyl Ether/Toluene)

This method demonstrates that taking the diethyl ether to dryness andgrinding the solid is not required for successful transformation to highquality microcrystalline α-alane. This reaction was performed followingthe above described procedure and apparatus, and produced approximately20-g batches of α-alane. A 1.0 M lithium aluminum hydride solution wasused. Approximately one quarter of the diethyl ether solution ofLiAlH₄/4AlH₃.Et2O/LiBH₄ was separated and reduced in volume under vacuumto remove ˜75-90% of the diethyl ether. The diethyl ether slurry wascombined with ˜110 ml of toluene. This slurry was subjected to a slightvacuum at ambient temperature (20-25° C.) to remove most of theremaining diethyl ether. After heating the solid mixture in mostlytoluene at ˜75° C. over 5 hours, the grey solid was rinsed three timeswith 50 mL of diethyl ether and dried under vacuum. The unpassivatedgrey solid weighed ˜6.0 g. In order to passivate the α-alane for longterm stability, the material was poured directly into a 1% hydrochloricacid solution. Sparks or evolution of hydrogen were not observed duringthe addition. The acid slurry was stirred ˜2 min and the grey solidisolated by filtration and washed with water and ethanol and air-dried.Yield was ˜5.3 g of a light grey solid. X-ray diffraction analysis ofthe passivated solid showed all α-alane with no evidence of otherpolymorphs or aluminum metal in the mixture. Differential scanningcalorimetry (DSC) and thermogravimetric analysis (TGA) both show thatthe passivated material, gives a single endothermic DSC peak atapproximately 168° C. and a weight loss of approximately 9.3% startingat 150° C.

Removing the diethyl ether to a minimal level while maintaining adiethyl ether slurry of the solid LiAlH₄/4AlH₃.Et₂O/LiBH₄ mixture allowsease of transfer of the solution on an industrial scale and removes theneed to grind the solid mixture as described in the previousexperimental descriptions.

Allowing a portion of the diethyl ether solvent to remain during theheating phase in toluene results in more control over particle sizedistribution, ease of passivation and higher hydrogen content of themicrocrystalline alpha alane.

The flowcharts and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems and methods according to various embodiments of the presentinvention. It should also be noted that, in some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. The descriptions are not intended to limit the scope of thetechnology to the particular forms set forth herein. Thus, the breadthand scope of a preferred embodiment should not be limited by any of theabove-described exemplary embodiments. It should be understood that theabove description is illustrative and not restrictive. To the contrary,the present descriptions are intended to cover such alternatives,modifications, and equivalents as may be included within the spirit andscope of the technology as defined by the appended claims and otherwiseappreciated by one of ordinary skill in the art. The scope of thetechnology should, therefore, be determined not with reference to theabove description, but instead should be determined with reference tothe appended claims along with their full scope of equivalents.

What is claimed is:
 1. A method for stabilizing aluminum hydride,comprising: producing aluminum hydride; placing the aluminum hydride ina nonaqueous solvent; adding a weak acid solution to the nonaqueoussolvent and allow the aluminum hydride to contact the weak acid solutionfor a period of time to form passivated aluminum hydride; separating thepassivated aluminum hydride from the solvent and weak acid solution; andwashing and drying the passivated aluminum hydride.
 2. The method ofclaim 1, wherein the nonaqueous solvent comprises an aromatic solvent, ahydrocarbon solvent, or mixtures thereof.
 3. The method of claim 1,wherein the nonaqueous solvent comprises toluene.
 4. The method of claim1, wherein the weak acid comprises an acid with a dissociation constantless than that of hydrochloric acid.
 5. The method of claim 1, whereinthe weak acid comprises an acid that partially dissociates in water. 6.The method of claim 1, wherein the step of forming passivated aluminumhydride further comprises cooling the weak acid solution and thenonaqueous solvent.
 7. The method of claim 1, wherein the step ofseparating the passivated aluminum hydride from the solvent and weakacid solution further comprises filtering the aluminum hydride from thesolvent and weak acid solution, washing the aluminum hydride with water,and washing the aluminum hydride with alcohol.
 8. The method of claim 1,wherein the step of drying the passivated aluminum hydride furthercomprises heating the passivated aluminum hydride.
 9. The method ofclaim 8, further comprising heating the passivated aluminum hydrideunder vacuum.
 10. The method of claim 1, wherein the aluminum hydridecomprises alpha alane.